Join Date: Jan 2010
Location: Splendora Texas
Bronco Info: 1989 4X4 351W C-6 3.54/3.55
The factory vacume-route diagram was missing off my ('89) truck when I bought it. Some of the line-colors have become faded over the years and I have since changed all the rubber hoses, so none of mine have the original color stripes.
Having said all that, I THINK that the plastic line you are describing goes to the plastic valve which sits next to the passenger-side (RH) exhaust manifold, which is part of the thermactor pump hose routing.
If that don't work, and if no one else pops in with a better answer, try to post a photo.
Hope this helps.
1989 Bronco 4X4 351W EFI C-6 3.54/3.55 Gears.